New research confirms that a week of hot tub sessions helps your body adapt to heat

A lot of health and performance advice falls into one of two categories: the ostentatiously miserable (deep tissue massage and exogenous ketones, say) or the suspiciously pleasurable (no, really, dark chocolate and red wine is all you need). Last summer, when I was mopping sweat from my insufficiently air-conditioned brow while working on an article about the performance-boosting effects of heat training, my topic felt like an example of the former. I chatted with an elite steeplechaser named Isaac Updike who had tried finishing up workouts three times a week with a 40-minute full-body soak at 104 degrees Fahrenheit (40 degrees Celsius). “There was always a time when you would think ‘Alright, I’m over this, how much time do I have left,’” he told me. “Sadly, the answer was always longer than you hoped.”

Now it’s winter, of course, and I feel almost self-indulgent writing about a hot-tub study—but here goes. The new study, published in Frontiers in Physiology, comes from Neil Walsh’s Extremes Research Group at Bangor University in Wales and was led by postdoctoral researcher Michael Zurawlew. It tested a fairly simple and accessible heat protocol, much like the one Updike tried: for six consecutive days, volunteers ran for 40 minutes on the treadmill then soaked up to the neck in a 104-degree hot tub for 40 minutes (or until they got uncomfortably hot or their core temperature hit 104 degrees). The result: a significant improvement in their body’s response to exercise in the heat.

As I discussed in the article last summer, there are a bunch of different reasons that repeated heat exposure might be useful. At the most basic level, it’s an extra stress on your body, ramping up the demands of whatever training you’re doing. As your body learns to cope with that stress, it makes adjustments that may be useful even in cool conditions. Your core temperature, both at rest and during exercise, gets a bit lower. You sweat sooner and more copiously. You sustain a higher volume of the blood plasma that helps ferry oxygen to your muscles.

As a result, heat acclimation protocols—exercise in the heat for at least an hour daily for a week or two, typically—are well-established. They boost your endurance in hot conditions, and possibly boost it in cool conditions too. Everyone knows this—at least in theory. In practice, one recent study found that only 15 percent of the track and field athletes at the 2015 world championships in muggy Beijing included heat acclimatization in their preparation. It’s expensive and inconvenient to fly somewhere for a hot-weather training camp, and it’s probably going to make your training worse. Finding a laboratory heat chamber to train in is, if anything, even less convenient and certainly less pleasant.

In recent years, there’s been rising interest in alternative like saunas and hot tubs. They’re a lot more accessible, and can be tacked onto the end of a normal training session. Walsh and his team published a previous study back in 2015 using the six-day 104-degree post-run hot tub protocol, and showed that it was effective at triggering heat adaptation, including a 4.9 percent improvement in 5K time in 91-degree heat.

The protocol takes advantage of the fact that simply going for a run in normal conditions elevates your core temperature, so hopping in the hot tub after the workout prolongs the period of time that your core temperature stays elevated. But this also raises a question: endurance athletes already raise their core temperature for prolonged periods every day anyway, which is why well-trained athletes are always considered partly heat-adapted. So do they get any further benefit from the hot bath? Or is this one of those tricks that works best on random undergrad volunteers but isn’t effective in actual athletes?

That’s the question that Walsh and his colleagues tackle in the new study. They enlisted eight serious endurance athletes who reported averaging nine hours of training per week, plus another eight “recreationally active” volunteers who reported three hours of endurance training per week. (That’s some pretty impressive recreational activity! But it’s not the same as the competitive athletes, whose ranks included an international marathoner with a lofty VO2max of 81 ml/kg/min.)

The study involved a whole bunch of different outcome measures: core temperature at rest, core temperature after a 40-minute submaximal run at 91 degrees, sweat rate, plasma volume, and so on. The overall picture was that six days of hot-tubbing helped pretty much everything except plasma volume, which increased by 3 to 4 percent, but with wide enough variability than the average increase wasn’t statistically significant. That’s not surprising, since it typically takes more than a week to see robust increases in plasma volume.

Here’s a graph that shows core temperature in Celsius (38.0 C is 100.4 F; 40 C is 104 F) at the end of the 40-minute hot run before and after the hot tub protocol:

(Photo: Courtesy Frontiers in Physiology)

It’s pretty clear that the runners in both groups are able to stay a bit cooler while completing the same run. That’s thanks to a set of adaptations to heat; for example, the body learns to send blood to the skin and initiate sweating earlier than usual to enhance cooling. Looking at the results overall, you could argue that the trained athletes might have a slightly smaller response compared to the recreational athletes, but any difference is minor and not statistically significant. The paper’s conclusion is that both groups get basically the same benefits, meaning that this is a technique that works for serious athletes too.

So who does this make sense for? The baseline sniff test for things like this is: if I really had 40 minutes to spare every day after my workout, would I get more bang for my buck by doing Crazy New Intervention #43 or by running (or cycling or whatever floats your boat) for another 40 minutes? Option B wins in the vast majority of cases. But say you’ve signed up for a marathon in Florida in January, and you live and train in the frigid tundra north of the Mason-Dixon line. Suddenly six days of hot tubbing looks pretty useful. It’s not an all-purpose miracle performance-booster, and as Isaac Updike found, it’s not as fun and relaxing as it may sound. Still, there are plenty of situations where a short bout of heat acclimation that doesn’t require plane tickets or rectal thermometers could be exactly what you need. Just don’t forget the red wine and dark chocolate.

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